Part 1. The Field of the Invention
This invention relates to novel, improved ink compositions and to writing instruments including such ink compositions. More precisely, the invention disclosed herein relates to aqueous ink compositions including water dispersible, polymeric shear-thinning providing materials and to writing instruments including the ink compositions.
Part 2. Description of the Prior Art
Ink compositions including water dispersible, polymeric, shear-thinning providing materials are disclosed in commonly assigned, copending application Ser. No. 693,930 filed Jan. 23, 1985, by Laura K. Case et al (now U.S. Pat. No. 4,671,691). U.S. Pat. No. 4,671,691 is expressly incorporated herein in its entirety by reference. Essentially, the ink compositions disclosed in U.S. Pat. No. 4,671,691 are thickened liquids at rest. They are non-Newtonian liquids and may have a rheological yield value and exhibit shear-thinning flow behavior or shear-thinning flow characteristics in use. In other words, they become thin, readily flowable liquids having a viscosity of no greater than about 100 cPs (or m Pa.S) at shear rates produced in writing such as with a ball-point pen. The ink compositions include at least one water dispersible, polymeric shear-thinning providing material uniformly dispersed in a highly polar solvent system which is preferably water or primarily water. Other materials such as coloring materials, surfactants, preservatives, corrosion inhibitors, humectants and the like can also be included in the inks.
Shear-thinning inks have relatively high viscosities at low shear rates and are susceptible or vulnerable to the effects of shock. Normally, the shear-thinning inks disclosed in U.S. Pat. No. 4,671,691 are contained in hollow, non-pressurized tubes and fed directly to the nib or ball of the writing instrument. However, when the ink is contained in non-pressurized tubes, the instrument should be handled carefully to avoid excessive shock. For example, if the instrument is dropped, tapped against a hard object or otherwise jarred, the column of ink in the tube may separate or be pulled or pushed away from the region of contact between the column of ink and the ball. This separation can result in a pocket of air being established between the ball and the column of ink which can affect the writing performance of the instrument until contact between the ink column and the ball is restored. Additionally, the contact between the column of ink and the ball can be broken by being replaced by a gas bubble caused by evaporation or by entry of air around an uncapped ball during periods of non-use.
As those in the art know, a high viscosity ink can be retained in hollow, pressurized tubes in order to control the effects of shock. However, pressurized tubes can present unattractive manufacturing cost features in terms of materials and assembly particularly for low priced, disposable writing instruments. This invention is addressed to the problems of controlling the effects of shock on high viscosity ink compositions and provides an especially effective solution to those problems.